This research project is designed to amplify ongoing studies on the host response toward tissue allografts. These studies are concerned primarily with examination of cellular and humoral mechanisms occurring during acute rejection of vascularized organ allografts in unmodified hosts and during their prolonged survival in enhanced recipients. The primary model involves the use of heterotopic cardiac allografts transplanted between inbred strains of rats. Elucidation of host mechanisms includes examining the nature and function of recipient cells infiltrating allografts, serial changes in lymphoid tissues of unmodified and enhanced recipients using histochemical stains, and the use of specific antiserum against various lymphocyte sub-populations and their effect on organ and skin allografts. The mechanisms underlying prolonged graft survival in enhanced recipients is under investigation, including the effect of various cell transfusions and the role of suppressor cell activity in induction and maintenance of immunological enhancement. A multifaceted investigative approach is used to allow us to obtain a more precise understanding of immunological events initiated by the host against foreign tissues.